“Oilon is the oldest ground source heat pump manufacturer in Finland.”
“In a borehole, for example, water temperature remains at a few degrees Celsius throughout the year. This keeps the heat pump’s coefficient of performance at a steady, high level even at the harshest winter conditions.”
https://oilon.com/en-gb/products-gb/ground-source-heat-pumps/
Geothermal [ground source] pumps typically cost $6,000 to $20,000, while air-source systems run between $4,500 and $8,000.
https://www.architecturaldigest.com/reviews/hvac/heat-pump-cost